Historically, storage or warehousing of goods or containers of goods has been effected on multilevel arrangements of shelves having limited depth and a great number of intermittent aisles for purposes of effecting reasonable accessibility for placement and retrieval of stored goods. For purposes of handling articles or goods of substantial weight, aisles of excessive width normally have been required for the purpose of permitting passage of heavy equipment of the type required for the transportation and placement of the goods. The end result of such systems is that a relatively small percent of a given size building or enclosure is usable as actual storage capacity.
More recently, efforts have been made to effect improvements in the storage density in a building or enclosure. In this respect, there has been a proliferation of what are termed stacker crane type units wherein loading and unloading devices are movable in aisles via a track or other means of controlled motion to afford transportation and loading and unloading from a plurality of storage spaces. These units are commonly capable of three dimensional positioning and are frequently automated for remote controlled operation. Although effecting some economies in space utilization, these units nevertheless generally require minimal depth rack configurations which are one or in the most advanced installations two positions deep adjacent the crane traverse aisles for purposes of accessibility for storage and retrieval of goods. This marginal space utilization combined with the necessary sophistication of a crane unit produces little overall cost advantage in comparison with conventional storage facilities which have been employed for many years.
Another approach which has gained some consideration involves the use of a plurality of movable pallets which are positioned in close horizontal proximity such that higher density storage is possible. In installations of this nature the pallets are normally movable in one or more directions to one or more service positions whereby an elevator or other device may be employed for retrieval of a particular pallet. Although affording vastly superior economies in terms of storage space per unit of building or enclosure space, these units have not in general enjoyed wide commercial acceptance. A primary problem encountered in conjunction with such systems is the expense produced by the necessity for a great proliferation of the apparatus required to effect movement of the individual pallets in two or more directions. In order to effect optimum space utilization, movement in perpendicular directions to shift the pallets to different positions is normally an absolute requirement. In addition, drive mechanisms are normally proposed for each of the individual pallets with these mechanisms duplicated in many instances to effect independent drive in two perpendicular directions. As a result of the duplication and proliferation of parts necessary to accomplish the requisite movements and driving force therefor, most such systems are characterized by substantial expense.
In addition to the above features, the prior art systems devised to provide pallet movement in two perpendicular directions on planar tracks normally employ wheels oriented in perpendicular directions. In order to provide unimpeded motion in the two directions the prior art has commonly resorted to the use of two different sets of wheels arranged on different vertical levels to engage intersecting multilevel tracks or rails. In other instances, efforts have been made to make at least one set of wheels vertically or pivotally movable such that they may be selectively brought into engagement or disengagement with intersecting rails when travel in one of two perpendicular directions is desired with a fixed set of wheels providing travel in the other direction. Either of these approaches presents certain problems, the former resulting in a lack of versatility and the latter in the use of numerous heavy-duty moving parts to effect the requisite vertical motion of pallets which are laden with extreme loads. In other instances, efforts have been made to provide for lateral motion between one set of wheels and the tracks upon institution of rolling of the other set of wheels on planar intersecting tracks. None of the known prior art devices have to applicant's knowledge provided a totally successful answer in terms of storage density and simple, reliable, and relatively inexpensive apparatus to effect pallet travel in a plurality of directions.